When you have ties to two countries, both might claim the right to tax the same income. A tax treaty divides up those rights: it assigns taxing authority for different income types (employment, pensions, dividends, royalties), sets reduced withholding rates, and provides tie-breaker rules for who counts as a resident.
Treaties don't eliminate tax — they prevent double tax and clarify the rules. Whether one helps you depends on the specific countries involved and the type of income; two countries may have no treaty at all, in which case you rely on each country's own double-tax relief.
Why it matters for your move
If you keep income, property, or a pension in one country while living in another, the treaty between them often determines your real tax outcome. It's worth checking before you assume a move is tax-neutral.