When breeders refer to conditioning their bettas they are referring to the period of time (usually 2 weeks) where they provide the best possible diet and housing conditions for their fish in an effort to prepare them for breeding. The diet is altered to encourage the females to produce eggs and for the males to store energy so they can sustain themselves through the rearing of fry.
For the most successful breeding to occur it is important to condition your bettas prior to introducing them to the same tank. The spawning process between betta fish can be violent and it is not uncommon for one or both fish to sustain injury. Conditioning your bettas with clean water and high protein foods will help them gain strength that will sustain them through the spawn and afterward when the male is focused on caring for the eggs and the female is focused on healing any wounds she may have suffered.
During the conditioning period check your water parameters daily.
Focus on a nutritious diet.
Near or at the end of your conditioning period, place your betta fish near each other where they can see each other. This will often stimulate their urge for reproduction. Look for signs they are ready to spawn. The male may begin building a bubble nest. The female will show her breeding stripes (vertical striping along her sides) and may begin to swell with eggs. These are all good signs that your fish are ready to spawn.