Normally this is something which applies to a specific person or is true for that person but may not apply to everyone or be true for everyone. It can also be applied to specific objects, events, or conditions.
Some people are allergic to certain foods. To say that a food is healthy is a relative truth, because that food may actually cause the death of certain people. Blueberries are an example. They are healthy for most people but not for some.
If you move to an area with many insects, it may be said that you should always wear insect repellent. This could also be considered a relative truth because it depends on the area where you live.
In addition, opinions are often considered relative truth. "The stars are beautiful" is an opinion, and so it is a relative truth. Perhaps some people think the stars are ugly.
Basically, anything that is not an absolute truth is probably a relative truth. Absolute truth is something which is true no matter the circumstances around it. "Tuesday is the day which comes after Monday" is an example. No matter who you are, if today is Monday then tomorrow is Tuesday.