In short: Tax reliefs are legal ways to reduce your taxable base or your tax if you meet conditions — for example for children, a young family's mortgage or voluntary pension contributions.
Simply put: A relief does not arrive by itself — you usually claim it in your annual return or through your employer. Each has conditions and a cap that change by year. A missed relief is not refunded automatically. A relief lowers tax; it does not make a purchase or investment better in itself.
Example: A €12,000 annual taxable base and €1,200 of voluntary pension contributions (within a 10 % cap). The base falls to €10,800 and the tax by €120.
General rule: Reliefs are national and change often.
Example: Bulgaria — the Personal Income Tax Act: children (Article 22c), a child with a disability (Article 22d), young families with a mortgage (Article 22a), voluntary pension contributions up to 10 % of the base (Article 19), contributions for missing service (Article 20). Only for people with no enforceable public debts (Article 23(2)).
Check in your country — Which reliefs exist this year and their conditions, at the tax authority.
Source: Personal Income Tax Act (Bulgaria): https://lex.bg/laws/ldoc/2135538631 · Your Europe — Taxes: https://europa.eu/youreurope/citizens/work/taxes/index_en.htm