A tax rate line is where a zone meets a class: it says what percentage items of that class pay when an order lands in that zone. The line has two names — one for your admin, and a display name your customer sees on orders and invoices.
Several lines on one combination
A zone-and-class combination can hold more than one line. India's intra-state GST is the classic case: a CGST line and an SGST line, each 9%, both on the same combination — the customer sees both on the invoice and pays 18% in total. Line names just have to differ within their combination.
Added versus compounded
Every line combines with its siblings one of two ways. An added line is calculated on the item price itself, and added lines simply sum — two 9% lines charge 18%. A compounded line is calculated on the item price plus what the other lines already added: a tax on the tax. Quebec's QST is the textbook example — it historically applied on top of the price including GST.
Combinations without a rate
If a zone-and-class combination has no line at all, items of that class sold into that zone get no tax from zone rates. The rates page counts how many combinations are covered and can list the unfilled ones, so nothing goes untaxed by accident — or stays taxed when you meant it to be zero: a deliberate 0% line and "no line" look the same to the customer, but the 0% line says you decided.