is this right .. how does it work ? 

thanks 

The same as spell slots, the warlock description says you have pact magic but then refers to to spells so I didn't know why they decided to call them pact slots here.

They function as you expect a warlock "spell" slot to. They are not split by level as all your spells are cast at the highest level you have and they come back after a short rest. 

ok so a first level warlock shoudl have two pact slots then since he knows two spells

No, at first level a Warlock knows 2 spells, and has 1 spell slot.

All but one of the previous posters have gotten a few things wrong, but it is a bit complicated so I will try to explain. The Warlock Table shows you how many slots you're supposed to have and what they level they are. You can view this here.

This site refers to the slots gained from Warlock as Pact Slots because they work a bit differently than the regular spellslots you get from other spellcasters. This distinction helps separate them on the character sheet and that's important because certain Warlock features, commonly certain Invocations, specifically require using Warlock spell slots instead of any other spellslots in the case of multiclassing. However, spells you know can be cast using any spell slot: be it the regular kind or Pact Magic kind, for they are, indeed, spell slots and are treated as such for any feature that refers to such.

The Pact Magic feature of the Warlock tells you how many spells you know, what levels of spells you have, when you can get more or better spells and so on.

A Level 1 Warlock will begin with one spell slot and know two spells of choice from the Warlock spell list. For example, you could get the Hex spell and Hellish Rebuke spells. You get to choose which spell you cast when you want to cast a spell and use up your spell (pact) slot.

Warlock spell slots work differently than normal spell slots in how they progress. You have fewer of them but you regain all Pact Slots at every short or long rest unlike most other spellcasters who need a long rest. Your spell slots will become higher in level as you progress in the class up to a maximum level of 5. By level 20 you will have 4x Level 5 Pact Slots, which means you can cast 4x 5th Level spells every short rest, which is quite a powerful feature compared to other spell casters, especially when you consider the Mystic Arcanum spells and your Eldritch Invocations.

I recommend reading the Pact Magic feature carefully. It gives examples and will detail how they work.