(in this verse) means,

The Cross; the wooden cross on which Jesus died for our sins.
In those days, people were nailed to huge wooden crosses as punishment for crimes and left there to die. Although He was innocent of ANY wrong-doing, this was the punishment Jesus took in order to pay the penalty for us, for all of our sins.

...Christ also suffered for us, leaving us an example, that ye should follow his steps: ,Who did no sin, neither was guile found in his mouth: ,Who, when he was reviled, reviled not again; when he suffered, he threatened not; but committed himself to him that judgeth righteously: Who his own self bare our sins in his own body on the tree, that we, being dead to sins, should live unto righteousness: by whose stripes ye were healed.
1 Peter 2:21-24