An open port means a service is listening on it and accepting incoming connections. Whether that’s a security risk depends on the service. Open port 80/443 (HTTP/HTTPS) on a web server: intended, not a risk. Open port 22 (SSH) exposed to the internet: risk if the SSH server accepts password auth, low risk with key-only auth. Open port 3306 (MySQL) or 5432 (PostgreSQL) exposed publicly: always a risk — databases should be behind a firewall or VPN. Scan any host with the port scanner to see what’s open. Compare against your intended service inventory; anything unexpected needs investigation.
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