What meristematic means
These are the cells the whole plant is eventually made from, and the only ones in it that are still dividing.
Meristematic tissue
Plant tissue made of cells that undergo active mitotic division and are able to produce new cells is called meristematic tissue. These cells are undifferentiated. Growth in plants happens because of the activity of meristematic tissue.
Undifferentiated
It means the cell has not yet become anything in particular. It has no thickened wall, no chloroplasts and no large vacuole, because those belong to cells that have taken up a job. A meristematic cell has kept all its options open, and the tissue it eventually becomes depends on where in the plant it ends up.
You have met the word once before
In grade 10, the page on tissue culture said that tissue is taken from meristematic places such as the apical bud, the lateral bud and the root tip. Those three places are exactly the three kinds of meristem in this skill. Tissue culture works because those cells are still dividing and have not committed to being one thing.
Where this sits relative to grade 10
The division involved is mitosis, which you met in the chapter on plant and animal cells: the chromosome number stays the same, and the two daughter cells carry identical genetic information. Nothing new happens at the level of the cell — what is new is that a plant keeps a supply of cells doing it, permanently, in particular places.
A plant never stops having embryonic tissue
An animal does most of its dividing early and then largely stops growing. A plant sets aside regions that go on dividing for as long as it lives, which is why a tree can add height and girth every year for a century. That difference is meristematic tissue, and nothing else.