The Septuagint is the Greek translation of the Old Testament, or the Tanach. It contains a few books now regarded as Apocryphal, which are often included in various Bible translations today.
It’s important because it gives us one more early manuscript tradition to go off of. It is in Greek, so it is a bit separated from our best Hebrew manuscripts, and translating a translation always adds some problems. But it gives us insight into the whole process.
It also gives us insight into how various groups of Jews and later Christians saw these texts. The Greek version does differ from the Hebrew, and that influenced where sects of Judaism, and Christianity, went with their theological thought.
So to sum up, the Septuagint is the Greek translation of the Old Testament and it’s important as it gives us an early manuscript tradition, and it gives us historic insight into second temple Judaism and early Christianity.
Side note: Jews dropped those “extra” books because they were written in Greek. Because of that, their manuscript tradition, which was followed by various Christian sects, also excluded them.
So what made them non-Canon was that they simply weren’t included in the manuscript tradition they received