What xP in a giveaway title mean? is it the points they get for it? Other questions.

Welcome to Steamgifts.
If you have not already done so, may I suggest you read the FAQ & guidelines. It is also a good idea to keep track of the announcements done by Steamgifts' admin.

Enjoy your stay at Steamgifts & good luck!

I see Oppenh4imer already pointed you to the FAQ and guidelines pages which are both excellent resources. To give a general outline of how SG works however and to answer your remaining questions about points and levels, here's a brief explanation.

When you join SG you are given 400 points with which to enter giveaways (this is the number preceding "P"). As you spend these points, they will regenerate so that you can enter more giveaways. Your total points will not exceed 400 however (barring exceptional circumstances outlined in the FAQ). All members are given the same number of points and they replenish at the same rate (I think it is 7 points per 15 minutes but I could be incredibly wrong about that). Giveaways each cost a certain number of points based on how expensive the game being given away is. No giveaway exceeds 50 points.

Level refers to your level here on SG. Everyone starts at level 0, and you can increase that level by giving away games. Full value games reward the most contributor value and increase your level the fastest, reduced value games (marked with an * when creating a giveaway) give 15% of the value of the game, and non-value games (marked with **) give no CV. Levels don't impact any of the functionality of SG but some people, when creating a giveaway, will limit who can enter by their level. So, for example, a giveaway creator may say that only people who are level 2+ can enter their giveaway. If you are level 0 or level 1, you will be unable to enter this giveaway (the green box you normally click will be colored red).

I hope that clarified your remaining questions, and welcome to SG!

If someone gets a key, they need to activate it on their steam account? or could they create another giveaway with it?

I'm surprised noone answered to this question yet. It's actually one of the most important concepts of the site. Big fat YES, you do have to activate the game you win on your Steam account (the one you are registered here with in case of having multiple). Failing to do so will get you temporary bans when caught, which will lead to a permanent suspension eventually when the behaviour continues. Creating another giveaway with a won game is called regifting and a big nono around here. When gifting games you gain Contributer Value (CV in short) which is defined by the P a game costs (to enter). This variable determines your level, and your level determines the availability of giveaways among others like groups, regional restrictions and people's whitelists. So FAQ and Guidelines, as has already been pointed out.

Welcome to Steamgifts.
In case you ever find your way back here, that is...