Fleshlight is a brand of male sex toys, specifically, a textured, sleeve-like device that replicates the feeling of penetrative sex. The term "Fleshlight" usually refers to the toy itself and can be used as a general descriptor for similar products by other brands.
"Cum" in this context is a reference to ejaculation, which is the climactic event during the act of masturbation or sexual intercourse.
So, the phrase "fleshlight-cum" likely refers to the act of using a Fleshlight toy for masturbation and reaching orgasm or ejaculation within it.