Capture literally means catching your pokemon. Catching a pokemon (except certain pokemons that could not be caught) requires the use of a pokeball which could be purchased from PokeMart.
How to catch[]
After entering battle and the selection of your perfect pokemon, click on your pokeball and click use item.
Pokeballs[]
There are 4 different types of pokeball in which are priced differently. (Refer to http://pokemoncrater.wikia.com/wiki/Pokemart) Each of these balls act as its value. The normal poke ball acts normally and the more you pay, the higher chance you could catch the target pokemon.