A Developmental Delay is when a child does not have the ability to perform a skill or behavior by the age they are expected to be able to perform the behavior. Children progress in their capabilities in a generally common order and timeframe. Skills that develop in this manner, such as walking and speaking, are called developmental milestones. These milestones occur in all healthy children at particular ages, with walking typically occurring between 9 and 16 months of age and communicative gestures (precursors to language) by 12 months. There's always some variability, but they are expected to perform these behaviors near these age ranges.
It is important to note that all children develop differently and reach milestones at varying times and with differing paces. If a child has not reached a milestone by an appropriate age then they may have a developmental delay. If a child appears to have a delay then they need to be examined and tested by a doctor or specialist to ascertain any problems that may be present in the child.
For example, a child who hasn't started speaking by 18 months may have a hearing problem that is prohibiting them from hearing the speech around them making them unable to replicate it. Although not all developmental delays signify an underlying problem it can be an important sign for identifying genetic or environmental issues.