echać is determinate, Jeżdzić is the indeterminate form of the verbs meaning 'to go by vehicle, ride, journey'.
This is simple basic slav logic in grammar and it is not just confined to the Polish Language or the very jechać or jeździć. The verbs of motion on foot are similar and are used in the same sort of way. Chodzić and iść are the verbs of motion to go on foot.
Ja pojadę jutro ku Londynowi-I will go to London tomorrow
and
Ja jeżdżę pociągiem każdy dzień do pracy-going to work like a commuter (backwards and forwards).
Ja idę do sklupu-I am going to the shop and
ja chodzę wszędzie pieszo-I go everywhere on foot.