We humans are more similar than we think. All people share 99.5% of their DNA sequence in common. Only the remaining 0.5% differs from one person to another. The DNA Matches are people who are likely to be your relatives (close or distant) because there are significant similarities between their DNA and yours within that 0.5%.
MyHeritage uses three different parameters to calculate DNA Matches: the total amount of shared DNA, the number of shared segments and the length of the longest segment. The algorithms and thresholds that are used to calculate matches might vary between the different providers, and therefore you might not see the exact same DNA matches in the different sites. For further details about these parameters, please read: How does MyHeritage calculate the potential relationship?
Please note that DNA Matches starting with 3rd cousins and going further share very little to almost no DNA with the user and could be the result of identity by state rather than identity by descent. This is a technical way of saying that these might be false positives and the DNA they share with is a result of coincidence.
The amount of DNA files that has been uploaded is growing every day, which means that the pool of possible matches will become larger and more diversified over time and users will receive more DNA Matches, possibly with closer relatives.