What it means
Pick the action that matters to you, count how many people took it, and divide by how many had the chance. That is your conversion rate. The action is your choice, and the number is meaningless until you have said out loud what it is.
Why it matters to a small business
Improving it is usually cheaper than buying more traffic. Doubling your visitors means doubling your ad budget. Taking a booking page from 4% to 6% costs nothing but attention, and it lifts every campaign you will ever run afterwards, including the free ones.
450 people reached your booking page last month and 27 of them completed a booking.
27 divided by 450, times 100
6%. The useful question is not whether 6% is good. It is what the other 423 hit that stopped them.
The bit people get wrong
Most arguments about conversion rate are really arguments about the denominator. Everyone who touched the site, everyone who reached the booking page, and everyone who started filling the form give three very different percentages for the same month. Write down which one you mean and keep using that one.