A focus chip will give you a "bubble" that protects you form losing concentration when using mindforce during combat.
Combat being when a mob is attacking you.
Each level of focus chip has a limite to the mount of focus charges you can get. Your skills determing how much of that chip limit woth of focus charges you get with each use.
The lessor focus chip has the lowest limit while the powerful one will give you a pretty high limit.
The focus charges will allow you to concentrate while using almost any mindforce chip except for TP chip.
IT will also protect you while sweating so you can sweat more efficiently.
The most common use for focus chips is to skill psychotrophy.
Psychotrophy skill being most needed for a TP chip, nerve blast and a wormhole chip.
Most focus chips are not worth all that much, especially the lower level ones. If you looted a medium to powerful one or higher then you can probrably get some ped for it. A lessor one would be pretty much TT food or close to it.