"Germline" is a word used in biology and genetics.
The germline of a mature or developing individual
are those cells that have genetic material that may
be passed to a child.
For example, sex cells, such as the sperm or the egg, are part of the germline.
But not just the sex cells; Because the sex cells got their genetic material from still other cells. The cells that produced sperm cells, and the cells that produced ovum, (called gametocytes,) are also part of the germline. And the cells that produced those cells are part of the germline- all the way back up to the zygote, the first cell that the individual came from.
But a cell in, say, your liver, is not part of your germline. No cell in your liver will ever make it to your children. Cells that are not part of the germline are called somatic cells.
If there is a mutation or other genetic change in the germline, then the change can be passed to offspring. But if a change happens in a non-germline cell, the change can't be passed to offspring.
"Germline" can also be used to refer to a lineage of cells spanning many generations of individuals; for example, the germline that links any living individual to the hypothesized first eukaryote of about one billion years ago, from which all plants and animals descend.