Accumulation refers to rolling up the values based on the previous records. In the chart expression,'No Accumulation' means that no rolling up will be made and the value of the expression will be displayed based on the current dimension values. 'Full Accumulation' means that the expression value will be shown as rolled up value for the current and previous dimension values.
say we have a chart that displays sales value for every year from 2009 till 2009. If 'No Accumulation' option is selected, then only the sales value for each year will be shown in the chart. If 'Full Accumulation' is selected, then yr 2000 will show only 2000's sales data and 2001 will show 2000+2001 sales and similarly all the next year sales value will be summed up with previous yr values and shown.
In case of accumulation by step, it does the accumulation based on the step defined.