Our genes contain instructions for making chemicals (proteins) that are used by our body to do stuff.

Our genes come in pairs - each member of a pair is called an "allele", which you can think of as a variant of the gene. Slightly different instructions for the proteins.

In some cases, a phenotype (an external characteristic) is determined by a single gene. Then, as you know, it might be that one allele is "dominant", and an alternative is "recessive". You only need one copy of a dominant allele to "express" that version of the protein, but you need two copies of the recessive one.

Here's one way that might work:

Suppose there's a version of the gene that produces a protein that makes a flower yellow. Without the protein, the flower will be white.

Let's call the allele that produces this protein Y.

Maybe there's a different version of Y which is "broken". The instructions it codes don't make the protein that makes flowers yellow. Maybe they make a protein that does something else, or maybe they make a protein that does nothing at all. Let's call that one y.

A plant might have two copies of Y, or two of y, or one of each.

If it has YY or Yy or yY, it's able to produce the protein, and so it has yellow flowers. The Yy ones might produce less of it, but that doesn't matter of the protein is like an "on/off" switch.

On the other hand, some plants have only yy, which means they can't produce the proteins, and so the flowers end up white.

So Y = "yellow flowers" is dominant, and y = "white flowers" is recessive, because the plant only needs one copy of Y to know how to make the protein that causes flowers to be yellow.