Isofix is how a car seat fits into the car. The car seat has metal points that clip onto the points in your car. A lot of brands do isofix seats as well as regular seats (britax, maxi cosy, m+p). Mother care can help you if you need more info!
Some car seats can have bases or iso fix bases too.... So basically there's two types of seat to choose - car seat alone (isofix or reg) or seat with base (either isofix or not)