A warm approach is a PUA term that described romantically approaching someone that one is already acquainted with instead of approaching a stranger.
Warm approaches seem to be the most common way people initiate romantic relationships (in societies with free-mate choice). Research indicates that most people (of both sexes) prefer to make romantic overtures to people they already know.
As incels are often socially isolated, the inability to make warm approaches, where their chances of success may be higher, may often be a large contributing factor to their inceldom. Supporting this, some studies have suggested that there may be a general tendency for women to wait longer to transition to sex when their partners are less physically attractive (and this may extend to other desirable traits).
These threads of evidence suggest that most men need to generally invest heavily in a woman (in terms of resources, time and entertainment value, in order to outwait her natural coyness. They need to build trust, generally present themselves as a suitable mate, outcompete other men for her affections, and take an indirect approach before she will accept a relationship with him and provide him with sex. This essentially means that lower-value men are often limited to warm-approaches, while men with high mate value may be able to effectively pursue short-term mating strategies more easily, as women will likely be more receptive to more aggressive and overt sexual advances on their behalf.