Please, help me with the meaning of the sentence: "Flowers in my room brighten it up"
The zero article here means any flowers at all in my room brighten it up.
You are not referring to any specific flowers, but to any and all flowers.
However, because the flowers in your room are part of the class of any flowers, you are indirectly referring to the flowers in your room.
It is like saying, Any flowers at all in my room brighten it up, including the actual flowers in my room.
If you want to talk specifically only about the actual flowers in your room, use the definite article or a similar word
The flowers in my room brighten it up.
Those/these flowers in my room brighten it up.