Measurement load is what percent of the instrument maximum internal sampling rate that the instrument use. A 100% measurement load means that the instrument pings as fast as it is able to. Consequently, a measurement load of 50% means that the instrument pings at half that rate. For example, an Aquadopp Current Meter has a maximum internal sampling rate of 23 Hz, which means that each beam will ping 23 times each second. An 8% measurement load will then mean that the instrument will ping each beam 2 times then go into wait state for the rest of each second. A lower measurement load thus allows the Aquadopp to spread out its averaging interval over a longer period of time as it save battery by going into a lower-power wait state during part of each second. The maximum internal sampling rate for each specific instrument can be found in the instrument technical specifications.
From signal theory we know that the more pings there are within an averaging period, the better the estimate of the true value we're measuring will get . A high ping rate will thus reduce the standard deviation. By increasing the measurement load, you can improve the precision of your measurements, but at the expense of battery consumption. By decreasing the measurement load it is the other way around. The exact relationship is shown in the Advanced tab in the Deployment planning menu. Just try and vary the measurement load, click on Apply and watch the change in the Battery utilization (for a specified assumed measurement duration) and the velocity precision. As you will see this is a trade-off between need for precision and need for duration.
Be aware that there is a difference between the midlife instruments (such as AWAC and Aquadopps) and Signatures. While the midlife instruments minimum have to ping at least once every second, and therefore has a restricted amount of possible measurements loads, the Signatures gives you the opportunity to use whatever measurement load you want and possibly spread the pings out over several seconds. You can therefore easily increase the average interval without this going on the expense of the power consumption. You get an overview over how many pings that are going into the average interval in the Effect tab in the Deployment Wizard.