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type="toggle"..what it means?/
<lightning:input type="toggle" label="Reimbursed?" name="reimbursed" class="slds-p-around--small" checked="{!v.expense.Reimbursed__c}" messageToggleActive="Yes" messageToggleInactive="No" onchange="{!c.clickReimbursed}"/>..can anyone explain me this
IN your case your code is say selected yes or no for Reimbursed value ..
When evenr user changes the value you are firing the onchange method