Mestizo is a term traditionally used in Latin America and Spain for people of mixed European and Amerindian heritage or descent.
When you refer to Latinas for example, those girls with nice booties, dark hair and olive skin, you are actually referring to mestizas (female mestizos)
Mestizo are people, who look half white and half Amerindian, famous people who are mestizos:
In other words, when you say Latino or Hispanic, you are actually referring to mestizo, this is why, by just looking at a girl, you are able to tell “she is Latina” because you notice that person has native american features (high cheekbones and almond eyes) mixed with European features (light skin, nose, mouth, skull shape). This is what makes you sure that she is Latina.
Not to confuse with the american idea of Mexicans, like these ones in the pic below, when Americans talk about Mexicans, they are referring to native Americans, people who look like this, these people are not mestizos (because they have little to none European blood), they are plain native Americans.