What was soft Reconstruction?

Although the term "soft" Reconstruction isn't usually used, historians do make a difference between the process used by both President Lincoln (and later President Johnson) and that used by Congress.
Simply put, both Presidents Lincoln and Johnson favored more lenient Reconstruction policies than did Congress. At the time, the Radical Republicans in Congress favored what many saw as a harsher approach.
In 1863, Lincoln offered the "10 percent plan," which would reinstate a Southern state into the Union after 10% of the state took a loyalty oath. Congress countered with the Wade-Davis bill which, if President Lincoln had signed, would have made readmission into the Union far more difficult.
After President Lincoln's death, President Johnson returned all of the property belonging to Southerners who pledged loyalty to the Union. He also allowed Confederate officials to apply for presidential pardons. By 1866, President Johnson had granted about 7,000 pardons to Confederate officials and the like.
Meanwhile, as mentioned above, Congress favored a harsher approach. It passed the Reconstruction Act of 1867, which divided the seceded Confederate states (except Tennessee) into 5 military districts. For more about Reconstruction, please refer to the links below.
http://www.digitalhistory.uh.edu/disp_textbook.cfm?smtID=2&psid=3103

https://courses.lumenlearning.com/boundless-ushistory/chapter/the-battle-over-reconstruction/

https://www.ushistory.org/us/35a.asp