SNMP has major versions. Each added security features.
SNMPv1 uses community strings for authentication. These are plain text passwords sent over the network. Never use v1 on untrusted networks.
SNMPv2c added better error handling and bulk operations but kept weak authentication. Faster than v1 but still insecure.
SNMPv3 added real security:
- Authentication with MD5 or SHA
- Encryption with DES or AES
- User-based access control
If your devices support SNMPv3, use it. If you must use v2c, restrict access by IP and use complex community strings.