Most quilters use a 1/4 inch seam allowance. Some miniature quilters use 1/8 inch. Whatever the size of your seam, it is most important that you remain consistent throughout your quilt.
In this photo, the triangle (bottom left) has been sewn to another triangle using a ¼ inch seam. It is a good idea to use a ruler to check your seam allowance when you first begin quilting as it is most important to be accurate. If you do not sew an accurate ¼” seam, your patchwork will not match at intersections.
In this Tree of Life block there are many seams. If you were off just a 1/16” when sewing each pair of triangles, this would cause you to come up short ¼” or more in each row.