Explain the condition called infant developmental delay.?

It refers to any. Condition (there are very many) that causes a child to mature, physically or mentally, at a slower rate than most other babies. There are growth delays, speech delays, cognitive (thinking) delays, gross motor (walking etc.) and fine motor (picking up small objects, etc.) delays.
Lags. There is a range of ages in months during which typically-developing infants attain a particular milestone in a developmental stream, e.g., communication, social-adaptive, motor.When an infant has not attained a milestone achieved by 90% of infants at a given age, he has a developmental delay. Depending on neurological findings, baby may be monitored for 1-2 mos. Or therapy may be instituted.
Not a condition. Developmental delay is just that delay in some part of infant development based upon the norms. There are many cause of the delay.