Apple's iMessage is the company's alternative texting service, and is the default service for anyone using an Apple devices. As with any messaging service, when using it you may want to quickly access a message or two that you've sent, either to send it again, or to copy it for use elsewhere.
One approach is to scroll through your message history, and manually select and copy the contents of a desired message from the previous bubble.
This approach is doable, and is perhaps easiest for retrieving a message when browsing far back through your history. However, if you are trying to access a more recent sent message, then you can do so from the Messages text entry box. Simply hold the Option key and press the Up arrow, and previously sent messages will appear ready to be selected or sent again. You can then continue to press the up or down arrows to see and select additional messages you have sent.
This feature is limited to the available conversation, so if you are searching for messages sent to one person, you cannot use this to access them from a conversation with another person. Additionally, it will only access the messages currently saved in your history, so if you have closed the conversation and have not otherwise saved it, then this feature will not work when you simply create a new conversation with that contact.
Despite these limitations, this feature may have its uses, one of which may be to retrieve a message to resend if your contact has deleted his or her history, or if you inadvertently send half of a message (perhaps with incorrect instructions or other information) and wish to retrieve the first half to finish it properly.