Full explanation
Your tech stack is the combination of software and services that make your product run. It usually includes a frontend, a backend, a database, hosting, and tools for payments and emails.
As a founder, you do not need to choose every piece yourself. What matters is to pick a stack that is reliable, popular, and easy to find help with.
A simple modern stack today: Next.js for the frontend, PostgreSQL for the database, Stripe for payments, and Vercel for hosting.
Concrete example
Your tech stack is Next.js, PostgreSQL, Stripe, Plunk for emails, and Vercel for hosting. That is enough to run a real SaaS.