The sex someone was assigned at birth. This isn't their 'real' sex or their 'true' sex. Most people are raised as the sex they were assigned at birth. As a phrase, this is a way to refer to the sex that was put on your birth certificate, without making assumptions about your actual/current sex, body or identity. In cultures with a gender binary, assigned sex is either male or female.
In some contexts, assigned sex may be referred to as "genetic", "biological" or "birth" gender. However, many members of the transgender community take offense to these and other similar terms, because it seems to imply that their identities are less real than those of cisgender people.
Assigned sex is also a useful phrase when discussing intersex people. Intersex people are often assigned a sex that does not match their chromosomes, genitals, and/or other characteristics of their body, and they may have been subject to non-consensual medical treatments to "normalize" their sex, often in childhood without their knowledge or permission.
Assigned sex is often referred to using the acronyms AFAB/AMAB (Assigned Female/Male At Birth), FAAB/MAAB (Female/Male Assigned At Birth), DFAB/DMAB (Designated Female/Male At Birth) or CAFAB/CAMAB (Coercively Assigned Female/Male At Birth). CAFAB and CAMAB are particularly used within the intersex community. Occasionally, UAAB (Un-Assigned At Birth) is used where a person was not assigned male or female at birth.