"Racial microaggressions are brief and commonplace daily verbal, behavioral, or environmental indignities, whether intentional or unintentional, that communicate hostile, derogatory, or negative racial slights and insults toward people of color." -"Racial Microaggressions in Everyday Life"
Microaggressions are often the result of implicit bias. Implicit bias is "when we have attitudes towards people or associate stereotypes with them without our conscious knowledge" (Perception Institute). For more information and resources on implicit bias, see Check Your Bias: Resources to Unlearn Implicit Bias.