I have downloaded some rextexture mods from the F3 Nexus and I wonder if they can work without the .esp file in FNV. It is only meshes and textures. What does the .esp excactly? What informations does it provide?

I believe in order to properly integrate the meshes or textures or both in-game, the esp is needed to "lock" it in place. Some retexture mods in F3 nexus require an esp because it uses a custom texture set. This practice is common to the author D7, also known as id2301. His sledgehammer retexture is a good example of that.

Some retexture mods might work without an esp, but it really depends on the content it has. Best check FO3Edit to know what it edits so you'd have a better picture of the situation.

I hope this helps

@njmanga097 Yes, it helps, thank you. I think I'm gonna try to port some F3 mods to FNV with the GECK. There is several tutorials on Youtube.
By the way, D7's mods are very good. His robot's textures are awesome, better than F4 ports.
I may use this topic again. See you.

EDIT: I do not know how I must attach the new .esp (from Fallout 3) to the FNV world in GECK. This is the last step of the port process and I do not understand how I have to complete it. Any help please?

@Prosp3ro You can't attach it because it uses Fallout 3 master file instead of New Vegas one. You have to replicate the contents of the .esp in NV's GECK.

@VWgolfR1 Well, I am not very familiar with the GECK. I think I am going to use the TTW mod instead of a port. Thanks for your help.

actually their is a process for changing a fo3 esp to be used in fnv, i wantched a youtube video i'll provide the link here, its really very simple, now after this you should be able to open up the esp in the fallout new vegas geck, but depending on what the esp is supposed to do you will have to use the ceck to edit it so that it will add the items or effect the esp file does. changing its master file does not mean it will automatically work in fallout new vega, thats why ive said you'll have to use the geck to add it into fallout new vegas. say its a leather jacket, well, the fallout 3 esp may say its found in megaton, fallout nee vegas don't have megaton, so you'll have to use the geck to place it into an area in the fallout new vegas world space, if not you'll not be adding the leather jacket to fallout new vegas. you'll just have a esp thats showing active but not adding the content, if you get my meaning on all this. here's a link to the youtube video so you can learn more about what i'm talking about.
https://www.youtube.com/watch?v=hPBT429LQ2Y

@bobbadan I already use this technique. It works very well with simple retexture mods, like for example Super Sledge Retextured by d7 aka id2301.
Thanks for the Youtube link, it could help anyone who tries to port mods.

your welcome, this process works for stuff like weapons, armors, and misc items but won't work for stuff like say a mod that changes a location in f03/fnv, like say it changes megaton, it wouldnt have any effect in fallout new vegas. it can help port some mods from one to the other, but won't work with all mod types. i just though i'd mention it to try and help everyone.