Semi-colons can be used to join two separate sentences that are on the same theme.
It was spring. The trees were beginning to blossom.
These sentences are fine as they are, but because the meaning of the second sentence is closely linked to the meaning of the first one, we can join them with a semi-colon.
It was spring; the trees were beginning to blossom.
It is not correct to join two sentences like this with a comma. This is a very common mistake.
It was spring, the trees were beginning to blossom. (wrong)
Semi-colons are useful for balancing two pieces of information.
My dad comes from Scotland; my mum is from Wales.
Jade's birthday is in March; Kayden's is in May.