Yes, when you purchase a STRNG subscription you also agree to have it automatically renewed, unless auto-renew is turned off at least 24 hours before the end of the current period.
You agree to your account being charged for renewal within a 24 hour period at the end of the current period and to this charge being the same as your initial fee, unless you opt for a different plan (e.g switching from monthly to annual).
You can manage your subscription at any time by logging into your account, or turn off auto-renewal in your account settings after purchase.