This is the 16078th most frequent Polish word.
ubierać
"Ubierać" means "to dress" or "to put on clothes."
The verb 'ubierać' is used reflexively here ('ubieram się') to indicate the act of dressing oneself in clothing suited to the weather conditions.
Ubieram się ciepło, bo na dworze jest zimno.
I dress warmly because it's cold outside.
Here 'ubiera' indicates that the subject (Mama) is putting clothes on another person (dziecko).
Mama ubiera dziecko przed wyjściem na spacer.
Mom dresses the child before going for a walk.
In this sentence, 'ubrać' is utilized metaphorically to signify 'decorating' the tree, rather than physically dressing it.