A Vedic mantra or the stanza of an ordinary poem is divided into four parts. In most metres there are four feet and each foot is divided into the same number of syllables or mantras. When the feet are not equal we have what is called a metre that is "visama": "vi+sama" = "visama". "Sama" indicates a state of non-difference, of evenness. When we do something improper, departing from our impartial "middle position", our action is characterised as "visama". The word is also used in the sense of "craftiness" or "cunning". But the literal meaning of "visama" is "unequal".
To repeat, if all padas of a stanza are not uniform they are said to be "visama". If alternate lines or padas are equal they are called "ardha-samavrtta". The first and second are unequal here, so too the third and the fourth. But the first and third and the second and the fourth are equal.
In most poems the padas are equal. Let me illustrate with a sloka with which, I suppose, all of you are familiar:
The four feet of this stanza:
1. Suklambaradharam Visnum
2. Sasivarnam caturbhujam
3. Prasannavadanam dhyayet
Each pada in this has eight syllables.
Only vowels and consonants in conjunction with vowels are to be counted as syllables; other consonants are not to be counted. Then alone will you get the figure of eight. The eight syllables in the first pada are :1. su; 2. klam; 3. ba; 4. ra; 5. dha; 6. ram; 7. vi; 8. snum. The other padas will have similarly eight syllables each.
The stanza with four feet, each foot of eight syllables, is "Anustubh", which metre is used in the Vedas and in poetical works of a later period.
For a general background, please see here