What is female ejaculation (squirting)?

Probably urine: The vaginal glands don't have significant muscle, so expelled fluid may be vaginal secretions expressed by a vaginal contraction, but more likely is urine because the female bladder neck/urethra are continent only at "low" pressures.

Squirting: “squirting” is controversial. Some indicate that it is a form of female ejaculation in which the fluid is propelled from para-urethral ducts during or prior to orgasm. Others hold that it is lubricating secretions or incontinence of urine.