It’s a “Monument to those who died in the struggle against fascism.”
It might be the most regularly defaced monument in the world as many monuments in Bulgaria are to the Soviet Red Army (- in fact, Bulgaria is the only country in Europe whose largest monument in the country, not this one but another, is to a foreign army). Here’s some of the most significant defacings:
Translation: “Long live Ukraine”
Translation: “In step with the times”
Translation (from Czech): “Bulgaria apologizes Prague ‘68” - because Bulgaria had soldiers march with the Red Army during the quashing of the Prague Spring revolution in 1968.
Other honoring Ukraine
Not sure what this says…