Cream of tartar is an ingredient you will commonly see on meringue recipes. However, it is absolutely replaceable if for any reason you can't find it. The cream of tartar doesn't add any flavour to the recipe, it simply adds acid that works to help stabilise the eggs. This helps to get really fluffy meringue.
When using cream of tartar use around 1/4 tsp for every 2 eggs in the recipe. If you are using lemon juice then double this and use 1/2 tsp for every 2 eggs.