Welcome back to Warhammer 40K 7th edition

I never played this, but apparently a rubber hose is an unwieldy AP2 weapon.

So, you're coming back to 40K after a long absence.  The last time you played was 5th edition, maybe earlier.  What's different?

Oh boy.  You'd better sit down.

6th edition was the biggest change 40K has had since the 2nd-3rd edition change, and 7th edition and newer army books took those changes further.  

Most is the same or similar.  Stat lines are the same, WS4 still hits WS3 on a 3+, power armor is a 3+ save.  But a lot is different.  If you're coming back to the game, take the time to read through the rules.  A number of things you may want to focus on:

• Premeasuring - remember having to actually guess ranges for your artillery?  No more.  You can measure anything at any time.  So if you want to be sure your melta gunner gets within 6" of the land raider when you move the squad - you can measure it while you're moving to be sure.  
• Movement by model - units still have to stay in coherency, but you can leave the guy with the Heavy weapon stationary to fire while others in the squad reposition.  No more one guy moves, whole squad counts as moving for shooting.
• Army construction - this is a doozy.  It's the wild west out there now with ways to build your force.  The old force org chart (1-2 HQ, 2-6 troops, etc.) is still there, under the name "Combined Arms Detachment" and is still the default way to build an army.  But you can use all kinds of detachments, formations and add-ons willy-nilly.  A formation is a collection of units, often with special rules attached.  Detachments are collections of units or even of formations, often with special rules attached.  Newer army books have unique detachments they can use and various formations.  A common detachment that everyone has access to is the allied detachment, letting you bring a few units from another army to fight along your main force.  You can also buy fortifications.  There are many, many other formations and detachments, and crazy combinations possible.
• 40K is bigger.  Big monsters are everywhere, and things previously only seen in Apocalypse games are a common sight.  
• Unit types - review these.  There are often special rules inherent with the unit type. With 40K being bigger, be sure to review the monstrous creature, flying monstrous creature, flyers, gargantuan monstrous creature and super-heavy vehicle rules.
• Shooting - review Snap Shot rules and Overwatch.  There are times when you can still shoot, but do so poorly, and some weapons are restricted.  You can shoot at units trying to assault you, but do so poorly.
• Wound allocation - hit closest models first, keep hitting them until they die, then move to next closest.  
• Assault - aside from Overwatch, some big changes:
• Random charge distances
• Many close combat weapons have AP values now, not "ignore all armor" like they used to.  Power weapons are not all the same, they have different effects and advantages depending on the type.
• Challenges - characters can seek each other out in the fight.
• Psychic powers - there is a separate psychic phase when you cast all your powers.  You don't take a leadership test to cast, you use psychic dice similar to how WHFB did it.  Want the power to go off?  Commit more dice to the attempt, but the more power you pour in the more likely  you'll have to deal with the Perils of the Warp.
• Vehicles - have "wounds" now, called Hull Points.  Bigger vehicles have more.  A glancing hit removes a hull point, a penetrating hit takes a hull point and rolls on the damage table.  The damage table is more forgiving.  Still a chance to one-shot a vehicle depending on your weapon and the vehicle.
• D-weapons - have a chance to just wipe you out with no save of any kind.  Super-heavy units have D-attacks, and they have started to creep into other places as well.  No matter how tough you make your character, there are things out there that can make them just disappear.

There are more, but I think those are the main things that would confuse a returning player.  Know that there are differences, adjust as you go and you'll do fine.