Since you originally tagged this with “Vietnam War,” I will use an example from that war to explain.
Primary sources are those sources that are written or otherwise created by people with firsthand knowledge of the events or topics that are being described. Secondary sources are sources that use primary sources to try to comment on the events or topics of a given time or place.
Looking at the link below, we can see both primary and secondary sources. The primary source is the excerpt from President Nixon’s actual speech. Nixon was speaking about a topic that was contemporary to him. He was speaking about a topic on which he had personal knowledge. His speech is clearly a primary source because he was an important player in the Vietnam War.
By contrast, the rest of the link below is a secondary source. It attempts to put Nixon’s speech in context and to explain to us how we should understand that speech. It is building on a primary source and commenting on it.
So, when reading, you should ask yourself whether the author of the source lived at the time and place being discussed. You should ask if they had personal knowledge of what they are talking about. If so, it is a primary source. By contrast, if the source merely comments on some topic at a later date, it is most likely a secondary source.
Be aware that it is hard to make clear distinctions about primary and secondary sources. For example, Nixon’s speech does comment on the past. It is a secondary source, for example, with regard to what President Eisenhower did in Vietnam. However, it is a primary source for the Vietnam War in the late ‘60s and for how that war was understood by one of the most important players in it.