The phrase "interpolation method" refers to a technique used to estimate or find values in-between known data points. It involves filling in the missing or unknown values based on the information available. Full definition
The delta method interpolates the General Circulation Model generally used in climate modelling at scales of 100 to 200 km using a thin plate spline spatial interpolation method to achieve the 30 arc seconds resolution [52]. (journals.plos.org)
Interpolation methods for Antarctic ice - core timescales: application to Byrd, Siple Dome and Law Dome ice cores. (usap-dc.org)
Cowtan and Way circumvent both problems by using an established geostatistical interpolation method called kriging — but they do not apply it to the temperature data itself (which would be similar to what GISS does), but to the difference between satellite and ground data. (realclimate.org)