According to modern scholarship and research, here is what is known about the Q source that Mark and Matthew were dubbed from:

Q stands for "Quelle," which is the German word for source.

Q is a hypothetical document that is believed to have been used by both Matthew and Luke in the composition of their Gospels.

Although no actual copy of Q has been found, many scholars are convinced that it once existed.

Q is considered one of the foundations of most modern gospel scholarship.

B. H. Streeter formulated a theory of Q in 1900, and it has since been widely accepted by many New Testament scholars.

Q is believed to contain sayings of Jesus that were not included in the Gospel of Mark.

The content of Q is thought to have influenced the composition of the Gospels of Matthew and Luke.

Evidence for the existence of Q
The evidence that suggests the books were composed from a source, such as the hypothetical Q document in the case of Matthew and Luke, includes:

The presence of verbatim agreements in wording between Matthew and Luke in passages not found in Mark.

The order of some shared material in Matthew and Luke that is different from Mark's order.

The theory that explains the existence of the Triple Tradition (material shared by Matthew, Mark, and Luke) and the Double Tradition (material shared by Matthew and Luke but not found in Mark) through a common source like Q.

Literary analysis of the Gospels showing patterns and structures that indicate sources beyond just oral tradition.

The variations in wording and details between Matthew and Luke in passages that are shared, suggesting independent but related composition.

The overall similarities and differences between the Gospels that can be best explained by a shared written source like Q.

These pieces of evidence, along with scholarly analysis of the texts, have led to the theory that there was a written source or sources used in the composition of the Synoptic Gospels.