It shows something we did in the past continuously or repeatedly. There are 3 ways to form the past tense:

1. Using the stem of the word in the simple present form and replacing the ending -ω with -σα. The verbs whose stem ends in a vowel have an ending in -σα and the vowel is transformed to -ού in the past form.

Γελάω -> Γελούσα = I was laughing

(εγώ) γελούσα
(εσύ) γελούσες
(αυτός/αυτή/αυτό) γελούσε
(εμείς) γελούσαμε
(εσείς) γελούσατε
(αυτοί/αυτές/αυτά) γελούσαν

Other examples following this rule:

Περπατάω -> Περπατούσα = I was walking
Αγαπάω -> Αγαπούσα = I was loving
Κολυμπάω -> Κολυμπούσα = I was swimming
Φοράω -> Φορούσα = I was wearing
Πονάω -> Πονούσα = I was hurting

2. Adding the prefix ε– to two-syllable verbs in order to form the past tense. In this case, the accent is over prefix -ε. The prefix is lost in the first 2 plural forms.

Παί-ζω ->Έπαιζα = I was playing

(εγώ) έπαιζα
(εσύ) έπαιζες
(αυτός/αυτή/αυτό) έπαιζε
(εμείς) παίζαμε
(εσείς) παίζατε
(αυτοί/αυτές/αυτά) έπαιζαν

Other examples following this rule:

Φέρ-νω ->Έφερνα = I was bringing
Γέρ-νω ->Έγερνα = I was tilting
Πλέ-νω ->Έπλενα = I was washing

3. The rarest form is using the stem of the verb in the present form and replacing the ending -ω with -α. This is most common in polysyllable words (more than three syllables) and in this case, the accent changes from the previous syllable in the singular form, whereas in the 2 first plural forms it keeps the intonation on the same syllable as in the present form.

Κα-θα-ρί-ζω -> Καθάριζα = I was cleaning

(εγώ) καθάριζα
(εσύ) καθάριζες
(αυτός/αυτή/αυτό) καθάριζε
(εμείς) καθαρίζαμε
(εσείς) καθαρίζατε
(αυτοί/αυτές/αυτά) καθάριζαν

Other examples following this rule:

Συμ-μα-ζεύ-ω -> Συμμάζευα = I was tidying
Κο-ροϊ-δεύ-ω -> Κορόιδευα = I was mocking
Κα-τα-φέρ-νω -> Κατάφερνα = I was achieving
Τα-ξι-δεύ-ω -> Ταξίδευα = I was travelling